F150 BRAKES

MY BACK BRAKES SEEM TO BE DOMINATE AND WANT TO LOCK UP. I PUT NEW WHEEL CYLINDERS ON IT AND CHECKED ALL OF THE PADS BUT NO HELP WITH THE PROBLEM. 2 MECHANICS HAVE LOOKED AT IT AND CAN'T FIND WHAT THE PROBLEM IS.

Check for new hardware, return springs etc. Them make sure the shoes with the longest face are on the rear facing portion of the baking plate. There are primary and secondary shoes, if they are switched, the brakes will do this, if all this is right, replace the proportioning valve.